The invention relates to a Wien filter provided with electrodes for generating an electric field, and magnetic poles for generating a magnetic field, said electrodes and magnetic poles being positioned around and having a finite length along a filter axis, and being positioned around the filter axis such that electric and magnetic forces induced by the respective fields and exerted on an electrically charged particle moving substantially along the filter axis at a certain velocity, take substantially an opposite direction to one another and are directed substantially perpendicular to the particle""s direction of movement through the filter, said filter having along its axis two ends determined by the finite length of the electrodes and magnetic poles, and said ends both being terminated by a closing plate which is positioned substantially transversely to the filter axis and is provided with an aperture around the filter axis to allow the particle to enter into and exit from the filter.
Such a filter is known from practice. The electric force exerted on the particle is caused by the particle""s electric charge and the electric field, and is directed in the same direction as or in a direction opposite to the electric field, depending on whether the particle""s electric charge is positive or negative. The magnetic force exerted on the particle referred to, the so-called Lorentz force, is caused by the particle""s charge, the particle""s velocity and the magnetic field, the direction of the magnetic force being perpendicular to the direction of the velocity and the direction of the magnetic field, while in addition depending on whether the charge of the particle is positive or negative. As the electric field and the magnetic field are directed perpendicular to one another in the filter, the electric and magnetic forces will be exerted on the particle in the same or in the opposite direction. The electric and magnetic field have to be directed such that the electric and magnetic forces are exerted on the particle in opposite directions. The magnitude of the two forces depends on the magnitude of the particle""s electric charge, the particle""s velocity and the magnitude of the two fields. For a specific ratio of the magnitudes of the two fields, the nett force exerted on the particle at a specific velocity of the particle will be zero. At this velocity a particle will not be deflected from its original trajectory, while particles that do not have this velocity, will be deflected. This is the principle on which a Wien filter is based.
However, for the filter to work properly, the ratio of the magnitudes of the two fields along the filter axis must be constant. If this is not the case, there will always be positions where a nett force is exerted on the particle, resulting in an undesirable deflection of the particle to be filtered, which will result in a deterioration of the filter""s efficacy. A well-designed shape and precise manufacture of the components of the Wien filter as well as a precise alignment, make it possible to obtain a constant ratio of the magnitudes of the two fields along the entire filter axis. The electrodes and magnetic poles may also affect each other""s field. In order to avoid this, the electrodes and magnetic poles may be made both electric and magnetic. However, all these are not easily achievable measures, and there is no guarantee that the precise alignment is maintained during use of the filter.
It is the object of the invention to eliminate the above-mentioned disadvantages. To this end a Wien filter is provided, characterized in that the closing plates are made from a material of low electric and magnetic resistance, and that the distance from the closing plates to a plane halfway along and perpendicular to the filter axis is at most approximately equal to the shortest distance from the filter axis to the electrodes and/or magnetic poles. Thus the closing plates determine the shape of the electric and magnetic fields in such a manner as to guarantee that the course of the two fields over the filter axis is identical, and that the ratio of the magnitude of the electric and magnetic fields over the filter axis is constant. The manufacture and assembly of the Wien filter according to the invention is simple. The requirements for obtaining an effective filter with respect to the precision of the components and their alignment in the filter, are modest. This makes the filter according to the invention both cheaper and more readily applicable than the known filters.
It will be obvious that in order to obtain the electric field, an electric charge has to be provided on at least two electrodes, and said electric charge on the two electrodes has to be different. The different electric charges may be obtained by applying a potential difference across the respective electrodes. To the expert it will also be obvious that in order to obtain a magnetic field, there have to be at least two different magnetic poles, that is to say a magnetic north and south pole. The magnetic poles are formed by magnets, of which a magnetic pole is directed toward the filter axis.
Preferably the aperture in the closing plates has such a small dimension that the distance from a point located substantially on the filter axis to the edge of the aperture in one of the closing plates is at most approximately equal to the distance from said point to the electrodes and/or magnetic poles. The screening effect of the closing plates on the two fields is then not affected by the aperture in the closing plates.
In a preferred embodiment the closing plates extend past the electrodes and magnetic poles. The closing plates may also be accommodated in a housing surrounding the electrodes and magnetic poles and made from a material with low electric and magnetic resistance. The two fields are now enclosed within the closing plates or the housing, so that their effect is confined to the interior of the filter. A housing is a better safeguard for this purpose.
Other advantages of the Wien filter according to the invention will become obvious from the following description of an embodiment of the Wien filter according to the invention and a possible application of the filter.